A liquid crystal display device is a planar display device having superb characteristics including high definition, thinness, lightweight, low power consumption and the like. Recently, the market size of liquid crystal display devices has been rapidly increased owing to improvement thereof in display performance, production capability and price competitiveness against other display devices. In a twisted nematic mode (TN mode) liquid crystal display device which is conventionally common, alignment treatment is performed such that the longer axis of liquid crystal molecules having positive dielectric anisotropy is generally parallel to surfaces of substrates and is twisted by about 90° between the upper and lower substrates along a thickness direction of the liquid crystal layer. When a voltage is applied to the liquid crystal layer, the liquid crystal molecules rise to be parallel to the electric field and thus the twist alignment disappears. In such a TN mode liquid crystal display device, a change of optical rotatory power which is caused by a change of alignment of the liquid crystal molecules by voltage application is utilized to control the amount of light to be transmitted.
Such a TN mode liquid crystal display device provides a large production margin and thus has superb productivity, but has a problem in terms of display performance, especially in the viewing angle characteristic. Specifically, when being observed in an oblique direction, the display plane of the TN mode liquid crystal display device exhibits a significantly decreased contrast ratio; more specifically, in the case where an image, which clearly exhibits a plurality of gray scale levels from black to white when being observed in a frontal direction, is observed in an oblique direction, the luminance difference between the gray scale levels is made significantly unclear. There is another problem that the gray scale characteristic of display is inverted; specifically, a darker area when being observed in a frontal direction appears to be brighter when being observed in an oblique direction (so-called gray scale inversion phenomenon).
Recently, as display devices having a more improved viewing angle characteristic than that of TN mode liquid crystal display devices, liquid crystal display devices of an in-plane switching mode (IPS mode), a multi-domain vertical alignment mode (MVA) mode, axially symmetric aligned micro-cell mode (ASM) mode, and the like have been developed. Liquid crystal display devices of these novel modes (wide viewing angle modes) solve the specific problems described above regarding the viewing angle characteristic, namely, the problems of the significant decrease of the display contrast ratio and the display gray scale inversion which are caused when the display plane is observed in an oblique direction.
However, as the display quality of the liquid crystal display devices is improved, a new problem of the viewing angle characteristic that the γ characteristic by observation in a frontal direction is different from the γ characteristic by observation in an oblique direction, namely, viewing angle dependence of the γ characteristic has become conspicuous today. The term “γ characteristic” means gray scale dependence of the display luminance. The γ characteristic in a frontal direction being different from the γ characteristic in an oblique direction means that the gray scale display state is different depending on the direction of observation. This problem is serious especially for displaying an image such as a photograph or the like or displaying TV broadcasting or the like.
According to a known method for alleviating the viewing angle dependence of the γ characteristic, two or more sub pixels are provided in one pixel, and the luminance of one sub pixel is made different from the luminance of the other sub pixel in display at an intermediate gray scale level (see, for example, Patent Document 1).
FIG. 18 is a schematic view of a liquid crystal display device 800 disclosed in Patent Document 1. In the liquid crystal display device 800, sub pixel electrodes 824s1 and 824s2 are connected to a common source line Ls via corresponding TFTs 830s1 and 830s2, and thus form capacitance coupling with corresponding storage capacitor lines CSa and CSb. In the liquid crystal display device 800, the storage capacitor lines CSa and CSb have different voltages from each other, and thus potentials of the sub pixel electrodes 824s1 and 824s2 are different from each other. As a result, the luminances of corresponding sub pixels S1 and S2 are different from each other. In this manner, the viewing angle characteristic is improved.
FIG. 19 is a schematic view of another liquid crystal display device 900 disclosed in Patent Document 1. In the liquid crystal display device 900, sub pixel electrodes 924s1 and 924s2 are connected to different source lines Lsa and Lsb via different TFTs 930s1 and 930s2. In the liquid crystal display device 900 also, potentials of the sub pixel electrodes 924s1 and 924s2 are different from each other, and thus the luminances of corresponding sub pixels S1 and S2 are different from each other. In this manner, the viewing angle characteristic is improved.
In a conventionally general liquid crystal display device, one color display pixel includes three pixels for displaying red, green and blue, which are the three primary colors of light. Recently, a display device including a color display pixel which includes four or more pixels for displaying different colors has been proposed. Such a display device is referred to as a “multiple primary color display device”. In a multiple primary color display device, the three colors of red, green and blue and also another color are used, and thus display can be provided in a wide color reproduction range (see, for example, Patent Document 2).
Patent Document 2 discloses a liquid crystal display device including a color display pixel which includes red, green, blue and yellow pixels. In the liquid crystal display device in Patent Document 2, the red, green, blue and yellow pixels are arrayed in two rows by two columns. The pixels are designed such that the red pixel and the blue pixel each have an area size larger than the size area of each of the green pixel and the yellow pixel.